The solar cell including a compound semiconductor such as a CdTe thin film and a CIGS thin film is growing in demand as an environmentally-friendly energy source. Conventionally, inexpensive soda lime glass produced by a float process has been used for the glass substrate of the solar cell. As shown in FIG. 1, in a CdTe solar cell, semiconductor thin films such as a CdS thin film 3 and a CdTe thin film 4 are formed on a glass substrate 10 with a transparent conductive film, and a metal conductive film 5 and electrodes 6 further are formed thereon. The glass substrate 10 with a transparent conductive film includes a glass substrate 1 and a transparent conductive film 2 formed on the glass substrate 1. In a CIGS solar cell, a Mo thin film for an electrode is formed on a glass substrate, and a CIGS semiconductor, an n-type semiconductor (a ZnO:Al film, for example), etc. are formed thereon.
In the processes of producing these thin film compound semiconductor solar cells, a high temperature treatment is needed. For example, a close space sublimation method is used for forming a CdTe thin film, and the temperature of the substrate may reach around 600° C. in this case. Moreover, it is possible to increase the efficiency of the semiconductor by applying a CdCl2 treatment at 620° C. to the formed thin film. In the case of a CIGS solar cell, the glass substrate is exposed to a high temperature of 500 to 600° C. in the step of forming a semi conductor film (a p-type optical absorption layer). In the both cases, a high temperature treatment at 500° C. or higher can enhance the photoelectric conversion efficiency of a thin film compound semiconductor solar cell.
However, since the conventionally-used soda lime glass has a strain point of around 500° C., a substrate made of soda lime glass is deformed at a temperature equal to or higher than that. Thus, when the soda lime glass is used for the substrate, the temperature of the substrate can be raised only to around 500° C. at highest in the high temperature treatments, causing a problem that the semiconductor does not exhibit the photoelectric conversion efficiency that it has intrinsically.
The CdTe semiconductor and the Mo electrode have a thermal expansion coefficient of about 50×10−7/° C., whereas the soda lime glass has a thermal expansion coefficient of about 90×10−7/° C. When, on a glass substrate made of soda lime glass, a thin film having a thermal expansion coefficient largely different from that of this substrate is formed at a high temperature of 500° C. or more, a high remaining stress is generated between the glass substrate and the thin film after they are cooled. In the process of producing a solar cell, conditions, such as the treatment temperature and cooling rate for the substrate, are selected so as not to cause problems due to the remaining stress. Thus, in the case where the thermal expansion coefficient of the substrate and that of the thin film are close to each other, the remaining stress can be reduced and the range of the selectable treatment conditions is widened. As a result, this allows for further improvement on the photoelectric conversion efficiency.
In this circumstance, there is needed a glass substrate for a solar cell that has a higher heat resistance (specifically, a strain point of 500° C. or higher), a smaller thermal expansion coefficient (specifically, an average thermal expansion coefficient of 75×10−7/° C. or less in a temperature range of 50 to 350° C.) and a large area, and is inexpensive, as compared to the soda lime glass substrate.
In recent years, solar panels have become larger in area, but it is preferable for them to be as light in weight as possible from the viewpoint of handling. For this purpose, it is desirable that the glass substrate for a solar cell has a smallest possible density.
Examples of the method for producing glass sheets with a large area include a down-draw process, a fusion process and a float process. The float process is superior to the other glass sheet production methods in that the float process makes it possible to mass-produce the glass sheets with a large area at lower cost. Moreover, glass sheet production apparatuses adopting the float process are widely used in the world to produce glass sheets for building materials. Thus, glass substrates that can be mass-produced by the existing float-process production apparatuses have an advantage that they can be supplied to a wide area.
The working temperature (the temperature suitable for forming glass sheets, and in the float process, it is the temperature at which the glass has a viscosity of 104dPa·s) of the soda lime glass is about 1000° C. In the float-process production apparatuses widely used to produce glass sheets for building materials, the heat and the erosion by the molten glass deteriorate more severely the bricks at the entrance of a tin bath as the working temperature of the glass is higher, exceeding 1000° C. Moreover, in the production by the float process, the glass needs to have a liquidus temperature lower than its working temperature. Taking into consideration that the working temperature should not far exceed 1000° C., the liquidus temperature of glass to be mass-produced by the float-process production apparatus preferably is 1200° C. or lower.
Patent Literature 1 discloses a glass composition having a higher strain point than that of conventional soda lime glass and a thermal expansion coefficient of about 50×10−7/° C. However, this glass composition contains 1 to 8 mass % of B2O3 and a large amount of the B2O3 is volatilized when the glass is melted, eroding severely the bricks used in a regenerator with a melting furnace. Accordingly, the melting furnace is deteriorated severely, causing a problem of higher cost.
Also, Patent Literature 2 discloses a glass substrate having an annealing point of 550° C. or higher as a glass substrate for a CIGS solar cell. However, this glass substrate contains 7% or more of an alkali metal oxide in total, and thus it is difficult, in reality, for the glass substrate to have an average thermal expansion coefficient of 75×10−7/° C. or less in the temperature range of 50 to 350° C.
On the other hand, glass substrates for a field emission display (FED), a plasma display panel (PDP) and the like also are required to have a high strain point. Patent Literature 3 discloses a glass substrate having a strain point of 590° C. or higher as such a substrate for a flat panel display. However, the glass composition disclosed in Patent Literature 3 contains a large amount of SrO and further needs a large amount of BaO (5 to 12.5% of SrO and 9 to 14% of BaO, according to claim 2). This increases excessively the density of the glass substrate (to 2.83 g/cm3 or more).